1. Field of the Invention
The present invention relates to a method of measuring the thickness of a semiconductor wafer and, more particularly, to a method of non-destructively measuring the thickness of the wafer.
2. Description of the Related Art
The final back-end processing steps of a semiconductor wafer commonly include the steps of bumping the wafer (adding solder bumps to the pads formed on the top surface of the wafer), and back grinding the wafer (removing a portion of the back side of the wafer to reduce the thickness of the wafer).
Before a wafer is accepted, the wafer is typically measured to insure that the thickness of the wafer falls within a specified range of values. One approach to measuring the thickness of the wafer is to dice the wafer (cut the wafer into a large number of individual die), and then measure the cross-section of a number of die.
A problem with this approach is that if the thickness of the die falls outside of the specified range of values, the die must be discarded. If the thickness of the die could be measured before the wafer is diced, a wafer that is too thick could be ground down some more until the thickness of the wafer falls within the specified range, thereby saving the wafer.
One approach to measuring the thickness of a semiconductor wafer before the wafer is diced is to use calipers. Calipers have a pair of probes, a control unit that adjusts the separation distance between the two probes, and a display unit that displays the current separation distance.
To measure the thickness of a wafer, the wafer is inserted between the two probes, and the control unit reduces the separation distance between the two probes until the two probes make physical contact with the top and bottom surfaces of the wafer. The displayed separation distance then indicates the thickness of the wafer.
One problem with utilizing calipers to measure the thickness of a semiconductor wafer is that, since the probes make physical contact with the top and bottom surfaces of the wafer, the probes can cause the wafer to crack. The probes can also smear the solder bumps, and scratch the surface of the passivation layer. Thus, there is a need for a non-destructive method of measuring the thickness of a semiconductor wafer.